1. Field of the Invention
The present invention relates to a process for fabricating a SOI substrate with excellent film thickness uniformity and suppression of the formation of film vacancies (or voids) and interface states. More particularly, this invention relates to a fabrication process for a SOI substrate which can be applied to high-functionality and high-performance electronic devices, highly integrated circuits, and so on, which are fabricated in a single-crystal semiconductor layer on a transparent insulator substrate of glass or the like or on a silicon substrate with an oxide film thereon.
2. Related Background Art
Formation of a single-crystal silicon semiconductor layer on an insulator is widely known as the Silicon on Insulator (SOI) technology, and much research has been conducted because this substrate has a lot of advantages that cannot be achieved by bulk silicon substrates which are used for fabricating ordinary silicon integrated circuits.
[SOS and SIMOX]
One of the conventional SOI technologies is the so-called SOS (Silicon-On-Sapphire), which is a method for hetero-epitaxially growing a silicon layer on a sapphire crystal, but the quality of the hetero-epitaxially grown silicon crystal is poor. Also, SIMOX (Separation-by-IMplanted-OXygen) is under practical use as a SOI forming technology for implanting a lot of oxygen ions into silicon and thereafter subjecting the resultant product to annealing, thereby forming an SiO.sub.2 layer with implanted oxygen being buried from the surface of silicon to the position of about 0.2 .mu.m. However, this implantation of many oxygen ions and annealing requires a lot of time, which is disadvantageous with respect to productivity and cost. The ion implantation also causes many crystal defects in the SOI silicon layer. Decreasing implantation of oxygen ions would make it difficult to maintain the film quality of the oxide layer. It is also considered to be difficult to change the thickness of the implanted SiO.sub.2 film layer.
[Bonding SOI]
Among the SOI forming techniques reported recently, there is "bonding SOI," with particularly excellent quality. This is the technology in which mirror surfaces of two wafers, at least one of which has an insulating film formed by oxidation or the like, are brought into close adhesion with each other. They are then subjected to annealing so as to reinforce the coupling of the adhesion interface. Thereafter the substrate is polished or etched from either side so as to leave a silicon single-crystal thin film having an arbitrary thickness on the insulator film. The most important point in this technology is a step for reducing the silicon substrate into a thin film. In more detail, normally, the silicon substrate, which is as thick as several hundred .mu.m or so, needs to be polished or etched uniformly down to the thickness of several .mu.m or even 1 .mu.m or less, which is technologically very difficult with respect to controllability and uniformity. There are roughly two ways for reducing silicon into a thin film. One of them is a method for carrying out thinning only by polishing (BPSOI: Bonding and Polishing SOI), and the other is a method for providing an etching stop layer immediately over a thin film to be left (actually, immediately under the thin film during fabrication of single substrate) and performing two stages of substrate etching and etching of the etching stop layer (BESOI: Bond and Etchback SOI). Since in the BESOI, a silicon active layer is often epitaxially grown over the preliminarily formed etching stop layer, this BESOI is considered to be advantageous to secure uniformity of film thickness. However, since the etching stop layer often contains a high concentration of impurities, it causes distortion of the crystal lattice, which causes the crystal defects to propagate to the epitaxial layer. There is also a possibility that the impurities diffuse upon oxidation of the epitaxial layer or upon annealing after bonding, thereby changing etching characteristics.
In these bonding SOIs, if there are contaminations in the bonding surfaces, or if there are asperities because the bonding surfaces are not flat enough, many vacant spaces called "voids" will appear at the bonding interface. From this view point, the BESOI discussed above is disadvantageous in many cases. The reason is as follows. The etching stop layer is normally formed, for example, by hetero-epitaxial growth by CVD or by epitaxial growth with doping of a high concentration of impurities. In the case of CVD, especially in the case of the hetero-epitaxial growth, the flatness achieved is often inferior to that of flat surfaces obtained by polishing. The etching stop layer is sometimes formed by ion implantation, but flatness is also degraded in this case.
[New BESOI technology]
An example of the technology for achieving good flatness of the bonding surfaces, uniform film thickness of the active layer as in the BESOI, and selectivity of etchback several orders of magnitude higher than that in the conventional BESOI is the technology for making the surface of silicon substrate porous by anodization, and epitaxially growing the silicon active layer thereon (Japanese Patent Application Laid-open No. 5-21338). In this case, the porous layer corresponds to the etching stop layer in the BESOI. However, since the etch rate of porous silicon is very high with a hydrofluoric acid based etchant as compared with single-crystal silicon, a high-selectivity etching characteristic is considered to be more important than the etching stop layer. Since this technology forms the porous silicon layer not by CVD, but by anodization of a flat single-crystal silicon substrate surface, the flatness of the epitaxially grown active layer is better than that formed in the BESOI in which the etching stop layer is formed by CVD or the like. The epitaxial layer growing on this surface achieves crystallinity nearly equal to that of an epitaxial layer grown on a non-porous single-crystal substrate. This enables us to use a single-crystal thin film equivalent to the epitaxial layer on the single-crystal silicon substrate with high reliability as an active layer, thus providing the SOI substrates with excellent crystallinity and with excellent film thickness uniformity.
K. Sakaguchi et al. reported that a substrate obtained by anodizing the surface of silicon single-crystal substrate to make it porous and effecting epitaxial growth thereon was bonded to a silicon substrate with an oxidized surface, the non-porous single-crystal silicon substrate portion was ground by a grinder to expose the porous layer, and only the porous layer was selectively etched with solution of HF/H.sub.2 O.sub.2 mixture, thus achieving 507 nm.+-.15 nm (.+-.3%) or 96.8 nm.+-.4.5 nm (.+-.4.7%) as a film thickness distribution of SOI silicon layer for 5-inch wafers. It is described that in the etching with the solution of HF/H.sub.2 O.sub.2 mixture in this case, the etching rate of the porous silicon layer is 10.sup.5 times greater than that of the non-porous silicon layer, and thus, the porous silicon layer functions well as an etching stop layer in the BESOI.
In addition to the method for bonding a single-crystal silicon substrate having a thermally oxidized surface or a transparent silica glass substrate to an epitaxial silicon film grown on this porous silicon, it is also possible to bond SiO.sub.2 surfaces of two substrates to each other. The interface state density of the interface between the epitaxial silicon film as an active layer and SiO.sub.2 (the thermally oxidized film of the epitaxial layer) is sufficiently low, and the thickness of the SiO.sub.2 layer can be controlled arbitrarily. Thus, the substrate can be fabricated as making full use of the characteristics of SOI. Then the surface of SiO.sub.2 at the bonding interface is activated by a plasma process, whereby the bonding strength can be enhanced and occurrence of voids can be suppressed.
The new BESOI technology described above permits us to obtain high-quality SOI substrates in which by the high selective etching of porous silicon layer the film thickness distribution preserves the flatness and film thickness distribution upon epitaxial growth. However, the above new BESOI technology has the following problems in removing the non-porous single-crystal Si region that was not made porous:
1. Problems raised by use of hydrofluoric acid based etchant of wet type.
The technology involves liquid exchange upon processing of many substrates it is difficult to control liquid concentration management. This causes poor productivity.
The SiO.sub.2 layer and SiO.sub.2 glass substrate are etched by the hydrofluoric acid based etchant because of the large etch rates thereof. Especially, in the case of bonding onto a transparent SiO.sub.2 glass substrate, the back face of the glass substrate is also etched, which degrades the transparency of the transparent substrate.
In the case of exposing the porous silicon portion with a wet-type etchant such as a hydrofluoric acid/nitric acid based etchant or alkali solution as a method for removing the non-porous single-crystal Si region, the etch rate of the low-density porous silicon layer is greater than that of non-porous silicon no matter what etchant is used. Because of this, before removing all of the non-porous silicon portion, etching will proceed faster in portions where porous silicon is exposed, so that variations of the remaining film thicknesses of porous silicon will become very large, several .mu.m or more. When the film thicknesses of porous silicon become as thin as several .mu.m or less, etching reaches even the underlying epitaxial silicon layer under the porous silicon, which degrades the uniformity of the film thickness of the final SOI layer. Therefore, the thickness of the porous silicon layer needs to be 10 or more .mu.m, and the porous silicon layer cannot be thinner than it.
2. Problem as to film thickness of the porous silicon layer:
In the case of grinding by a grinder as a method for removing the non-porous single-crystal Si region, a thickness of not less than 10 .mu.m is necessary as a grinding margin to stop grinding at the underlying porous silicon layer as well as a damage layer due to grinding, and therefore, the porous silicon layer cannot be made thinner than it.
Therefore, a lot of time is necessary for formation and etching of the porous silicon.